Without limiting the scope of the invention, its background is described in connection with amplifiers constructed of NPN-type bipolar transistors. For normal operation, this implies positive base-emitter voltage, positive current into the base terminal, positive collector-emitter voltage, and positive current into the collector terminal.
Small, light-weight, high-efficiency power amplifiers are essential to portable self-powered radio-transmitting equipment such as battery-operated handheld cellular telephones. Efficiency is a figure of merit which quantifies the amount of DC input power required to produce a specified output-signal amplitude. Efficiency translates inversely to power consumption under specified operating conditions. High efficiency is therefore necessary to minimize required battery capacity.
High-efficiency power-amplifier transistors are fabricated using many different basic technologies such as GaAs FET, silicon bipolar, and GaAs heterojunction bipolar. Most of these technologies are compatible with monolithic circuit-integration techniques; allowing resistors, capacitors, diodes, microstrip transmission lines, and other structures to be implemented on the same substrate as the transistors. This circuit-integration capability is another asset when the size and weight restrictions of portable equipment are considered.
A high-efficiency power-amplifier design seeks to maximize the ratio of signal-power output to DC-power input. High-efficiency power amplifiers are usually operated at conduction angles of 180 degrees or less (Class-B or Class-C). The conduction angle refers to the portion of one period of a sinusoidal input signal over which the transistor is "on", or conducting. A full period of the input signal contains 360 degrees.
The signal-frequency behavior of an amplifier operated under large-signal (nonlinear) conditions is dependent upon both the fundamental-frequency and the harmonic-frequency response of the circuits that surround the transistor. Design of the fundamental-frequency impedance of the circuit which couples signal power to the output is based primarily upon the amplifier transistor's desired operating voltage and current. Presenting the correct load impedance to the transistor's output sets the voltage/current ratio for large-signal operation.
High-efficiency transistor performance also requires specific terminations at integral multiples (harmonics) of the signal frequency. Of great importance is the circuit behavior at two and three times the signal frequency. For example, it has been demonstrated that the second-harmonic voltage present at the transistor's collector terminal should be minimized. It has also been demonstrated that amplifier performance is enhanced when a non-zero third-harmonic voltage is present at the transistor's output terminal.
Most nonlinear power amplifiers, especially those operating Class-B or Class-C, generate significant second-harmonic currents in their output circuits. In order to minimize the second-harmonic voltage present at the amplifier-transistor collector terminal, the surrounding circuits are typically designed to present a low impedance to the device at twice the signal frequency.
The optimum third-harmonic output voltage is nonzero, and typical third-harmonic current is relatively small (compared to second-harmonic current). As a result, the surrounding circuitry may attempt to maximize third-harmonic output voltage by presenting a high impedance at three times the signal frequency.